6210_NewGloucester_2017-2018_AnnualReport_Web

SABBATHDAY LAKE | 2017 Water Quality Report

DISSOLVED OXYGEN & TEMPERATURE Sabbathday Lake has a history of DO depletion in deep, cold areas of the lake (below 8 meters) in late summer. In 2017, DO levels dropped below 5 ppm at 15 meters in July, 9 meters in August, and 9 meters in September (Figure 3). As in previous years, the most significant DO depletion occurred in September, with DO readings reaching <1 ppm at 15 meters. An oxygen bubble between 4 and 7 meters, accompanied by a marked decrease in water temperature, is indicative of the thermocline. The upper part of the thermocline is where algae are most productive; below this, light penetration, and thus algae growth, is limited. In addition, cold-water fish prefer waters of 18 ° C and can tolerate waters up to 24 ° C. In summer, the upper 4-6 meters are too warm for cold-water fish, restricting desirable habitat of cooler, oxygenated waters to between 5 and 9 meters depth. Continued monitoring of DO is needed to characterize oxygen depletion over time in Sabbathday Lake. Collecting regular

FIGURE 3. Dissolved oxygen (top) and temperature (bottom) profiles collected in 2017 at Sabbathday Lake.

phosphorus samples near the bottom of the lake could also provide information on internal phosphorus cycling, a process triggered by low DO that releases phosphorus from bottom sediments to serve as fuel for algal growth. CHEMICAL PARAMETERS Color Since 1982, color in Sabbathday Lake has ranged from 5 to 32 PCU, with an all data average of 13 PCU. Sabbathday Lake has low color compared to average color in Maine Lakes and has shown a relatively stable trend in color over the sampling record (Figure 4). In 2017, color in Sabbathday Lake ranged from 11 to 14 PCU, with an average of 12 PCU (Table 1). 2017 color was 1.2 PCU lower (better) than the all data average. Alkalinity Since 1982, total alkalinity in Sabbathday Lake has ranged from 7.1 to 17.5 ppm, with an all data average of 10.8 ppm. Sabbathday Lake has slightly lower alkalinity compared to average alkalinity in Maine Lakes and has shown a relatively stable trend in alkalinity over the sampling record (Figure 4). In 2017, total alkalinity in Sabbathday Lake ranged from 11 to 12 ppm with an average of 11.7 ppm (Table 1). 2017 total alkalinity was 0.9 ppm higher (better) than the all data average. Per the USEPA, these average total alkalinity levels in Sabbathday Lake fall in the “sensitive” category of 10 ppm to less than 20 ppm. A lake is well-buffered against

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